Ion exchange is the reversible interchange of ions between a solid (often termed resin) and a liquid in which there is no permanent change in the structure of the solid that is the ion exchange material. The utility of ion exchange rests with the ability to use and reuse the ion exchange materials, employing an appropriate regeneration step.
There is an enormous market for high purity water. It is used extensively in industrial applications, with notable major users including the pharmaceutical, electronics and power generation industries.
In the final treatment (polishing step) for most applications, dissolved salts are commonly removed from the water by passage through ion exchange materials or resins. A combination of a cation exchange resin (in the H.sup.+ form) and an anion exchange resin (in the OH.sup.- form) are used to remove the cations and anions, respectively. Two beds, one containing each resin type, may be used in tandem for the removal of many salts. If the aqueous solution is first passed through a bed of cation exchange resin (in the H.sup.+ form), the cations in the solution are taken up by the cation exchanger, and an equivalent amount of H.sup.+ ions art released into the solution (thus preserving electroneutrality). This (now acidic) solution is then passed through a second bed of anion exchange resin (in the OH.sup.- form), and the anions in the solution are taken up by the anion exchanger. The OH.sup.- that is released neutralises the H.sup.+ in the solution thereby forming water. After passage through the two beds, the dissolved salts have effectively been removed from the solution. (If the sequence of the desalting is reversed, a corresponding scenario, with an intermediate alkaline solution, will apply).
For a number of water purification applications, a mixed bed of ion exchange resin is preferable, either in the place of the tandem bed configuration, or in addition to it, as a final polishing step. In this case, the cation and anion resins are intimately mixed, and the cations and anions are removed from the solution at the same time, with immediate neutralisation of the H.sup.+ and OH.sup.- released by the resins forming water. It is possible to achieve a greater level of salt removal from aqueous solutions with this configuration, as the chemical equilibrium process is "driven" by the loss of H.sup.+ and OH.sup.- by mutual neutralisation.
In either configuration, once the resins are exhausted (i.e. once there is no more resin in the H.sup.+ and OH.sup.- forms), the cation and anion beds must be regenerated. For the conventional chemical regeneration processes, the cation and anion resins in the mixed resin bed must be physically separated before chemical regeneration can be carried out. This regeneration is usually by treatment with acid (typically H.sub.2 SO.sub.4) and alkali (typically NaOH), respectively. This chemical process involves the storage and handing of concentrated acid and caustic solutions, as well as the disposal of the effluent regeneration and wash solutions after use. This is undesirable in the point of view of cost, safety and environmental considerations.
Other methods not requiring harsh chemical treatment alone have been developed for the regeneration of ion exchange resins. Some of these methods include electrodeionisation systems utilising specific ion-permeable membranes and chemical treatment solutions. Often the membranes are expensive and can to leak or rupture during use. Furthermore, the apparatus needed to house the resins with the membranes can be expensive to produce and maintain. Other methods available are adapted only to regenerate cation or anion form of resin, thus requiring two systems to regenerate both forms of ion exchange material.
The present inventors have developed a method of treating ion exchange resins using mild, inexpensive and ecologically acceptable electrochemical techniques. This method does not require regenerating chemicals, and has the added advantage that the water purification process and ion exchange regeneration can be carried out in the same vessel, with no need to disturb the resin bed. In the case of the mixed bed systems, this process also has the advantage that the regeneration can be carried out on resin beds without needing to separate the cation exchange resin from the anion exchange resin.